Porous silica is commonly used as a matrix material for chromatographic separations. With surface areas in the neighborhood of 300 meter(m)2/gram(g), commercially available chromatographic grade silicas possess a relatively high surface area. Mesoporous materials, which typically possess surface areas in excess of 1000 m2/g and even as high as 1600 m2/g, are commonly used as adsorbents, catalysts, and catalytic supports. With such high surface areas, these materials should provide superior separating ability as a chromatographic matrix in liquid chromatography (LC), flash liquid chromatography (FLC), and high performance liquid chromatography (HPLC). Mesoporous inorganic oxide particles differ from conventional porous inorganic oxides in that their surface areas are significantly larger than those of conventional porous inorganic oxides.
Various techniques exist for synthesizing mesoporous silica. For example, U.S. Pat. No. 4,554,211 to Arika et al., discloses a technique for synthesizing mesoporous silica spheres using an emulsion templating mechanism in basic solution. Other patents describing techniques for synthesizing large pore oxides in basic solution include U.S. Pat. No. 5,068,216 to Johnson et al., U.S. Pat. No. 5,168,828 to Degnan et al., and U.S. Pat. No. 5,308,602 to Calabro et al.
Recently, processes have been developed for synthesizing mesoporous silica spheres in acidic solution. In an article by Stucky et al., Oil-water Interface Templating of Mesoporous Macroscale Structures, Science, 1996, 273, 768-771 an emulsion process for synthesizing mesoporous silica spheres was described. A silicon alkoxide (TEOS) was dissolved in an organic solvent, typically mesitylene. This mixture was added, slowly over a period of 30 minutes, to an aqueous acidic solution containing a cationic ammonium surfactant (CTAB). Stucky found that by varying the stir rate during the course of the reaction, the particle morphology could be changed. At slower stirring rates, the reaction mixture produced fibers, and as the stirring rate was increased, the amount of fibers decreased with the increasing amounts of spheres. It was shown that the size of the spherical particles decreases with increasing stirring rates. Scanning Electron Microscopy (SEM) indicated the particles were hollow and spherical in nature. It was shown that these hollow spheres were brittle, and could be crushed with a spatula. The brittle nature of the spheres, in combination with the fact that they were not porous throughout their interior, seemed to indicate unfavorable characteristics for their use a chromatographic matrix.
Qi et al., in the article Micrometer-Sized Mesoporous Silica Spheres Grown Under Static Conditions, Chemistry of Materials, 1998, 10, 1623-1626, describes the formation of mesoporous silica spheres by a process using a cationic-nonionic surfactant mixture in aqueous acidic conditions. A typical synthesis involved stirring an aqueous acidic solution of a cationic ammonium surfactant (CTAB), and a nonionic surfactant (decaethylene glycol monohexadecylether), to which an alkoxysilane was added (TEOS). This material was presumably porous throughout its interior, although this was not specifically addressed in the article. The material seems to possess desirable characteristics, a high surface area (1042 m2/g) and ˜5 micrometer (μm) particle size, for use as a chromatographic matrix, but the long synthesis time (16 hours) and the use of a mixture of surfactants rather than one does not seem desirable for use on a commercial scale.
Yet another process for synthesizing mesoporous silica spheres in acidic aqueous solution is described by Ozin et al., in the article Synthesis of Mesoporous Spheres Under Quiescent Aqueous Acidic Conditions, Journal of Materials Chemistry, 1998, 8(3), 743-750. An acidic aqueous solution consisting of an alkoxysilane (TEOS) and a cationic ammonium surfactant (CATCl), was allowed to react under static conditions for a period of 7-10 days at 80° C. It was also demonstrated that spherical particles could be synthesized at room temperature with a modified reaction mixture. Particle sizes appeared to range from 1-30 μm. While the spheres Ozin et al. produced are monodisperse, the lower surface area (750 m2/g) and long synthesis time (7-10 days) makes the material and process unattractive for use on a commercial scale. Monodispersity refers to the degree to which the particles are uniform in size and shape, and must take into consideration the percentage of spherical particles, the size range of these particles, and their percent interconnectivity.
All of the processes described above produce materials which exhibit regular powder X-ray diffraction patterns with one or more relatively narrow diffraction peaks. This indicates that they contain a relatively ordered arrangement of pores. It appears that the materials produced by these processes are similar to SBA-3, a mesoporous material with a hexagonal arrangement of linear pores (“Mesostructure Design with Gemini Surfactants: Supercage Formation in a Three-Dimensional Array”, Huo et al., Science, 1995, 268, 1324). SBA-3 is similar to the more widely known MCM-41, which has an identical arrangement of pores but is synthesized in basic solution (“Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Templating Mechanism,” Kresge et al., Nature, 1992, 359, 710). While mesoporous silica having such ordered pores has use in a variety of contexts, the processes for synthesizing such materials tends to take longer, or be more complex, than is commercially desired.
Spherical Mesoporous silica particles have been produced using a reaction mixture including fluoride. See “Spherical MSU-1 Mesoporous Silica Particles Tuned for HPLC,” Boissière, C.; Kummel, M.; Persin, M.; Larbot, A.; Prouzet, E. Adv. Funct. Mater. 2001, 11, 129-134. Such particles have relatively small pore volumes (0.45 cm3/g) and require long synthesis times (48-72 hours).
Spherical mesoporous silica particles have also been produced using a reaction mixture including ethanol. See “Counterion Effect in Acid Synthesis of Mesoporous Silica Materials”, Lin, H.-P.; Kao, C.-P.; Mou, C.-Y.; Liu, S.-B. J Phys. Chem. B. 2000, 104, 7885-7894. Such particles are produced using a basic, as opposed to acidic, reaction mixture without the use of fluoride and require long synthesis times (6-48 hours).
U.S. Pat. No. 6,334,988 to Gallis et al., the disclosure of which is incorporated herein in its entirety, discloses a method of making mesoporous silicates from an acidic reaction mixture having a mineral acid, an inorganic oxide source, a surfactant, and water. However, the pore volume of the mesoporous silicates produced by such a method ranges from 0.35 cm3/g to 0.75 cm3/g. Further, the heating temperatures required to achieve greater than 90% spherical particles range from 110° C. to 210° C. It is desirable to have mesoporous inorganic oxide spherical particles having larger pore volumes and larger pore diameters than was previously achievable and methods of making such particles. It is also desirable to be able to produce mesoporous inorganic oxide spherical particles with desirable properties in a shorter period of time than in prior methods.